The present invention provides an improvement to a spindle motor useful in an information storage system in which ball bearings are utilized to journal a rotor about a shaft. The embodiments of the present invention are designed to incorporate a magnetic bearing and a multi-pole spindle magnet disc in a reduced height flat spindle motor to improve motor performance. Such motors are especially useful in information storage systems such as disc drives.
Overall disc drive size is of paramount concern to the disc drive design engineers. As smaller, light weight, portable computers have grown in demand, the disc drive engineer has continually searched for ways to reduce both the overall size and weight of the disc drive while maintaining or improving overall drive performance. FIG. 1 shows a flat spindle motor in the prior art developed by the assignees of the present invention. In the prior art, conventional ball bearings 10 were utilized to journal the rotor assembly 12 about the stator 14.
However, the use of conventional ball bearing assemblies in conjunction with high rotational speed devices has evidenced problems which are deleterious to drive system performance. Specifically, conventional mechanical bearings used are subject to metal wear, vibration/shock, have higher acoustics and friction problems. In order to alleviate the problems associated with conventional mechanical bearings, magnetic bearings may be substituted, thereby improving overall system performance. The use of magnetic bearings to improve disc drive system performance was disclosed in the concurrently filed United States patent application Ser. No. 08/201,676 entitled "Passive Magnetic Bearings for a Spindle Motor" by Dunfield et al., and is expressly incorporated herein by reference. However, the multi-pole configurations disclosed therein are optimized for stability rather than size considerations and are thus not ideally suited for flat spindle motor applications.
Spindle motors utilizing permanent and electromagnets in combination are also well known in the art. Specifically, the combination of a permanent magnet attached to a rotor, interacting with a separate electromagnet assembly attached to a fixed stator, as a means of journaling a disc about a stationary shaft, is well known in the art. In the prior art, spindle permanent magnets served only as a means for rotating the rotor about the stator via an axial pivot. Separate means for maintaining the physical relationship between the rotor and the stator, namely conventional ball bearing systems, were previously employed. These separate means required extra space, additional piece parts, lubricants and contributed to the overall manufacturing cost of the final product. In order to eliminate these separate means, magnetic bearings integrated with the spindle magnet of a spindle motor may be substituted, thereby minimizing piece parts and optimizing size considerations. The use an integrated spindle magnet and magnetic bearing to improve disc drive system performance was disclosed in the concurrently filed United States patent application Docket No. A-58554 entitled "An Integrated Passive Magnetic Bearing System and Spindle Magnet for Use In Spindle Motor, U.S. Ser. No. 08/201,798" by Dunfield et al., and is expressly incorporated herein by reference.
However, the integrated spindle magnet and magnetic bearings disclosed therein were oriented axially opposing the stator winding laminated stacks. Accordingly, a single pole magnetic bearing integrated with the spindle magnet provided sufficient radial support for the spindle motor. In the flat spindle motors of the prior art, the spindle magnet is oriented perpendicular to the axis of rotation, in order to minimize the overall height of the spindle motor. As such the single magnet pole configuration disclosed in Dunfield et al. '554 may not ordinarily be sufficiently stable in the radial direction.
Additionally, a mechanical bearing provides axial support, helping to hold together the rotor and stator in an axial direction in the event of motor inversion. Accordingly, the removal of the mechanical bearings requires other means of maintaining the relative axial positions of the spindle motor rotor and stator assemblies. Dunfield et al '554 and '697 utilize multi-pole magnetic bearings in attraction mode as well as the attraction of the rotor and stator steel to the magnetic bearings in order to maintain axial orientation or pre-load. As described previously, flat spindle motor design is not ideally suited for multi-pole bearings due to size optimization goals. Unfortunately, single-pole magnetic bearings may not provide sufficient pre-load because of their small size.
Finally, disc drives are designed to sustain mechanical shocks. This is especially true in flat spindle motor applications for uses in portable computers and disc drive systems. Mechanical bearings maintain a hard contact interface between the stator and the rotor assembly and thereby provide a rigid interface. Accordingly, shocks received by the stator are transferred to the rotor and conversely. In order to minimize these shocks, shock mitigation means may be employed to dampen the shocks seen by the overall unit, without concern for the individual rotor or stator assembly. However, the use of magnetic bearings raises different problems. Since the mechanical rigid interface between the stator and rotor no longer exists, ordinary shock mitigation means which dampen the shocks to the entire motor assembly are no longer sufficient. This is because without the hard mechanical interface between the rotor and stator assembly found in a mechanical bearing configuration, the magnetic bearings tend to isolate the rotor from any mitigation or dampening that conventional shock mitigation means provide. Unfortunately, magnetic bearings exhibit very low dampening characteristics. As such, a magnetic bearing would ordinarily provide insufficient dampening of shocks received, and thereby deleteriously affect spindle motor operation in the high shock environments ordinarily associated with flat spindle motor applications.